1. Field of the Invention
The present invention concerns a field distribution correction element for positioning on an examination subject in a magnetic resonance system for local influencing the B1 field distribution during a magnetic resonance acquisition. Moreover, the invention concerns a method for generation of magnetic resonance exposures of an examination subject in which a corresponding field distribution correction element is positioned on the examination subject for local influencing of the B1 field distribution, as well as the use of the field distribution correction element for homogenization of a B1 field in a magnetic resonance system.
2. Description of the Prior Art
Magnetic resonance tomography is a widespread technique for acquisition of images of the inside of the body of a living examination subject. In order to acquire an image with this method, i.e. to generate a magnetic resonance exposure of an examination subject, the body or the body part of the patient to be examined must initially be exposed to an optimally homogeneous, static basic magnetic field (designated as a B0 field) which is generated by a basic field magnet of the magnetic resonance measurement apparatus. Rapidly switched generation devices for spatial coding are superimposed on this basic magnetic field during the acquisition of the magnetic resonance images, these gradient fields being generated by gradient coils. Moreover, RF pulses of a defined field strength are radiated with a radio-frequency antenna into the examination volume in which the examination subject is located. unwanted variations in the acquired magnetic resonance signal that can adulterate the measurement result.
The RF pulses disadvantageously exhibit an inhomogeneous penetration behavior in conductive and dielectric media (such as, for example, tissue) precisely at the high magnetic field strengths of the type that are inevitably present in magnetic resonance tomograph due to the required basic magnetic field B0. This leads to the situation that the B1 field can vary strongly within the measurement volume. Special measures therefore must be taken in order to achieve an optimally homogeneous distribution of the transmitted RF field of the radio-frequency antenna in the entire volume, in particular in examinations known as ultra-high field magnetic resonance examinations, in which modern magnetic resonance systems with a basic magnetic field of three Tesla or more are used.
A simple but effective approach to solve the problem is to modify the electrical environment (namely the dielectrical) of the examination subject in a suitable manner in order to compensate for unwanted inhomogeneities. For example, dielectric elements with defined high dielectricity constant ∈ (preferably ∈≧50) can be positioned in the examination volume for this, for example directly on the patient or at the patient. For example, the typical RF field minima occurring in magnetic resonance examinations of a patient in the chest and abdomen region can be compensated by placing corresponding dielectric elements, which compensate the minima by causing a local increase in the penetrating radio-frequency field, on chest and abdomen of the patient.
For example, distilled water decanted into a plastic film bag can be used as such a dielectric element. Unfortunately, the use of such “dielectric cushions” filled with water has the unwanted side effect that they are visible in the magnetic The magnetic flux density of these RF pulses is typically designated B1. The pulse-shaped radio-frequency field is therefore generally called a B1 field. The nuclear spins of the atoms in the examination subject are excited by these RF pulses such that they are deflected from their equilibrium state (which is parallel to the basic magnetic field B0) by an amount known as an “excitation flip angle” (also called “flip angle”). The nuclear spins then precess around the direction of the basic magnetic field (B0. The magnetic resonance signals thereby generated are acquired by one or more radio-frequency acquisition antennas. The acquisition antenna can be either the same antenna with which the radio-frequency pulses are radiated, or a separate acquisition antenna. The magnetic resonance images of the examination subject are generated on the basis of the acquired magnetic resonance signals. Each image point in the magnetic resonance image is thereby associated with a small body volume (known as a “voxel”) and every brightness or intensity value of the image points is linked with the signal amplitude of the magnetic resonance signal acquired from this voxel. The correlation between a radiated RF pulse with the field strength B1 and the flip angle α produced thereby is given by the equation
                              α          =                                    ∫                              t                =                0                            τ                        ⁢                          γ              ·                                                B                  1                                ⁡                                  (                  t                  )                                            ·                              ⅆ                t                                                    ,                            (        1        )            wherein γ is the gyromagnetic ratio (which can be considered as a fixed material constant for most nuclear magnetic resonance examinations) and τ is the effective duration of the radio-frequency pulse. The flip angle produced by an emitted RF pulse, and thus the strength of the magnetic resonance signal, consequently also depend on the strength of the radiated B1 field in addition to the duration of the RF pulse. Spatial fluctuations in the field strength of the excited B1 field therefore lead to resonance exposures. Therefore a “dielectric cushion” with a filling containing a relaxation agent is proposed in DE 10 2004 015 859 A1. By the addition of the relaxation agent, it is ensured that the protons in the dielectric cushion relax more quickly and therefore are not registered in the image data acquisition. The cushion is virtually invisible in the image. Such cushions, however, have an unwanted influence on the B0 field. The homogenization of the transmitted RF field is additionally not yet optimal. In DE 10 2006 025 940 it is therefore proposed to introduce the relaxation agent into the filling of the dielectric cushion such that it is bound to particles separated from one another. The introduction of free charge carriers into the dielectric element is thereby largely avoided, and the conductivity of the filling of the dielectric cushion is clearly reduced. This leads to a reduction of the shielding effect and thus in total to a significantly stronger homogenization effect.
A further disadvantage of cushions with low-viscosity (i.e. liquid) contents (such as, for example, distilled water) is that these cushions are uncomfortable in handling and to an extent are even unsuitable if, for example, the shape of the cushion is altered by gravity. High requirements with regard to density and rigidity are then additionally posed on the material of the cushion enclosure. For these reasons, the fillings in the two types of cited cushions are fashioned as gels and are thereby more form-stable and better usable. They additionally exhibit all desired properties such as a good homogenization of the transmitted and acquired RF field, an invisibility in MR images, and in a biological interaction by the employed fillings. However, an optimized cushion size of approximately 35 cm×25 cm×4 cm results for these types of dielectric cushions, and the weight of the filling is approximately 3.5 kg. A cushion thickness of 4 cm is an essential parameter for effectiveness of the cushion.
Both the relatively large thickness and the high cushion weight have disadvantages. Stressing of the abdominal cavity with a heavy cushion is often perceived by the patient to be uncomfortable. Moreover, given the use of an additional local coil, the separation (spacing) of the appertaining local coil from the patient distinctly increases due to the cushion located between local coil and patient's body, so the signal-to-noise ratio of the acquired signals is poorer.